Recently, there have been increasing demands for improvements in sensitivity and image quality in silver halide photographic light-sensitive materials. With the aim of meeting these demands, a large number of studies have been made particularly on the morphology, production, etc. of silver halide crystals.
Above all, tabular grains of silver halide have been actively investigated by many researchers since they are favorable for sensitivity improvement. Particularly, as disclosed in Japanese Patent Examined Publication No. 38692/1988 and Japanese Patent Publication Open to Public Inspection No. 14636/1986, tabular silver halide emulsions comprising silver iodobromide grains wherein silver iodide is localized are drawing much attention as having excellent properties.
The localized silver iodide described above is produced by supplying a water-soluble silver salt and a water-soluble halide into an aqueous solution containing a protective colloid by the double jet method or the triple jet method and depositing them on silver halide crystals.
Japanese Patent Publication Open to Public Inspection No. 58237/1987 and Japanese Patent Examined Publication No. 38692/1988 disclose a production method of producing a silver halide emulsion comprising twin crystals either by adding a portion of the silver iodide to be deposited in an aqueous halogen solution and carrying out a conversion reaction with a highly soluble halide or by adding a portion of the silver iodide to be deposited as such and depositing it on silver halide crystals.
Japanese Patent Publication Open to Public Inspection No. 153428/1977 discloses a production method for a silver halide emulsion comprising twin crystals in which silver iodide crystals are first formed, and then silver ions and bromide ions are supplied by the double jet method and a silver iodobromide emulsion is formed.
Japanese Patent Publication Open to Public Inspection No. 183644/1989 discloses a production method for a tabular silver halide in which fine grains of silver iodobromide crystal prepared outside the reactor are supplied to the reactor and deposited on the seed crystal.
As stated above, there have been a variety of methods proposed to introduce iodide ions into silver halide crystal or to localize iodide ions in the crystal.
Meanwhile, with respect to grains of tabular twin crystal, the prior art includes a tabular twin crystal emulsion with uniform grain size, described in Japanese Patent Publication Open to Public Inspection No. 14636/1986, and another tabular twin crystal emulsion wherein the relative standard deviation among the grains is not more than 20%, described in Japanese Patent Publication Open to Public Inspection No. 209445/1987.
However, these methods do not offer a satisfactory improving effect, and it is expected that a new silver halide emulsion and a method of its production will be developed.